FIG. 1 shows a cross-section of a typical example of conventional InGaAs planar photodiode. On one surface of an n.sup.+ -type InP substrate 1, an n-type InP buffer layer 2, an n.sup.- -type InGaAs light-absorbing layer 3, and an n.sup.- -type InP window layer 4 are formed in a stack in the named order with the layer 2 contacting the substrate 1. A p-type impurity, such as Zn, is diffused from a portion of the surface of the n.sup.- -type InP window layer 4 to reverse the conductivity type of portions of the window and light-absorbing layers 4 and 3 and to thereby form a reversedconductivity type region, i.e. p.sup.+ -type region 5. The bottom of the p.sup.+ -type region 5 extends into the n.sup.- -type InGaAs lightabsorbing layer 3. A broken line 12 in FIG. 1 represents the front of a depletion layer 3a. In the illustrated case, the front 12 substantially provides a p-n junction between the p.sup.+ -type region 5, and the n.sup.- -type InP window layer 4 and the n.sup.- -type InCaAs liqht-absorbing layer 3.
A surface protecting insulating film 6, such as a silicon nitride (SiN) film, is formed by, for example, plasma CVD, to cover the n.sup.- -type InP window layer, leaving uncovered at least the portion which provides a light receiving area 11. A positive electrode 7 is formed in ohmic contact with the p.sup.+ -type region 5 within the opening in the insulating film 6 where the light receiving area 11 is provided. Also, a metallic light-blocking film 8 is formed over the insulating film 6, with a gap 9 disposed between the positive electrode 7 and the light-blocking film 6 to provide electrical insulation therebetween. On the opposite surface of the n.sup.+ -type InP substrate 1, a negative electrode 10 is formed in ohmic contact with the substrate 1.
In this planar photodiode light incident on the light receiving area 11 passes through the window layer 4 and a substantial portion of the light is absorbed by the light-absorbing layer 3, in particular, by the depletion layer 3a. Carriers 31, indicated by dots, generated by absorption of light by the depletion layer 3a, are accelerated by the electric field within the depletion region 3a. Thus, the carriers provide a drift current component which responds very quickly to an input light signal and is detected as a light-responsive electric signal flowing between the electrodes 7 and 10.
Light incident on other portions, such as the gap 9 between the electrode 7 and the light-blocking film 8 of the device is absorbed by portions of the light-absorbing layer 3 other than the depletion layer 3a, which generates carriers 32 indicated by small circles. The carriers 32 diffuse and reach the depletion layer front 12 or p-n junction and, therefore, are included in the detection current as a diffusion current component. The diffusion current component is generated due to the spatial density gradient of the carriers 12 and therefore the speed of movement of the carriers 32 which provide the diffusion current component is much slower than the carriers 31 which provide the drift current component. This speed decreases the response time of the device to the input light signal.
A photodiode which is free of the above-described drawback, i.e. a slow response to an input light signal, of the conventional device shown in FIG. 1 is shown in, for example, Japanese Published Patent Application No. SHO 55-140275. FIG. 2 shows a cross-section of a major portion of a photodiode shown in this Japanese application. The same reference numerals are attached to similar portions of the photodiodes of FIGS. 1 and 2, and they are not described further. A positive electrode 71 makes an ohmic contact to the p.sup.+ -type region 5 within the opening in the insulating film 6 where the light receiving area 11 is provided, as in the photodiode of FIG. 1. A portion of the positive electrode 71 extends beyond an opening 23 in a wire-bonding region to provide an extension 72. An insulating film 21 of, for example, phosphosilicate glass (PSG) is disposed over the insulating film 6 and the positive electrode 71. A metallic light-blocking film 22 is formed over the insulating film 21. The opening 23 for wire bonding is formed through the insulating film 21 and the light-blocking film 22. A connection wire is bonded to the positive electrode 71 in the opening 23.
In the planar photodiode shown in FIG. 2, the bottom of the opening 23 for wire bonding is closed by the extension 72 of the positive electrode 71, and, therefore, light incident on the opening 23 is entirely blocked Accordingly, only light that is incident on the light receiving area 11 passes through the window layer and reaches the depletion layer 3a in the light-absorbing layer 3, and, accordingly, only carriers 31 that substantially contribute to the generation of a drift current component are generated. Generation of carriers in the portions of the light absorption layer 3 other than the depletion layer 3a is avoided and therefore, substantially no diffusion current component is generated Accordingly, the reduction in responsed time which otherwise would be caused by a diffusion current component is prevented.
Although the response degradation attributable to a diffusion current component can be avoided, the planar photodiode of FIG. 2 has a disadvantage in that it requires additional steps for forming the insulating film 21 the metallic light-blocking film 22, and the opening 23 for the wire bonding. The addition of such manufacturing steps raises manufacturing costs. Another disadvantage of the structure of FIG. 2 is that a large parasitic electrostatic capacitance is formed between the metallic light-blocking film 22 and the positive electrode 71 which increases the time constant of the device. An increase in the time constant decreases the response speed of the output of the device to the input light signal.
It is, therefore, desired to have a semiconductor photodetector device which is free of the above-described disadvantages of conventional devices by completely blocking light incident on portions of the device other than a predetermined light receiving area, preventing a light-blocking film from increasing parasitic capacitance, and also avoiding increasing the number of the manufacturing steps relative to the FIG. 1 device.